From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934507Ab3E1PCV (ORCPT ); Tue, 28 May 2013 11:02:21 -0400 Received: from e23smtp08.au.ibm.com ([202.81.31.141]:33455 "EHLO e23smtp08.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934439Ab3E1PCS (ORCPT ); Tue, 28 May 2013 11:02:18 -0400 Message-ID: <51A4C6F1.9000607@linux.vnet.ibm.com> Date: Tue, 28 May 2013 23:02:09 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: Marcelo Tosatti CC: gleb@redhat.com, avi.kivity@gmail.com, pbonzini@redhat.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch References: <1369252560-11611-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1369252560-11611-5-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <20130524203432.GB4525@amt.cnet> <51A2C2DC.6080403@linux.vnet.ibm.com> <20130528001802.GB1359@amt.cnet> In-Reply-To: <20130528001802.GB1359@amt.cnet> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13052814-5140-0000-0000-00000346BDB8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/28/2013 08:18 AM, Marcelo Tosatti wrote: > On Mon, May 27, 2013 at 10:20:12AM +0800, Xiao Guangrong wrote: >> On 05/25/2013 04:34 AM, Marcelo Tosatti wrote: >>> On Thu, May 23, 2013 at 03:55:53AM +0800, Xiao Guangrong wrote: >>>> Zap at lease 10 pages before releasing mmu-lock to reduce the overload >>>> caused by requiring lock >>>> >>>> After the patch, kvm_zap_obsolete_pages can forward progress anyway, >>>> so update the comments >>>> >>>> [ It improves kernel building 0.6% ~ 1% ] >>> >>> Can you please describe the overload in more detail? Under what scenario >>> is kernel building improved? >> >> Yes. >> >> The scenario is we do kernel building, meanwhile, repeatedly read PCI rom >> every one second. >> >> [ >> echo 1 > /sys/bus/pci/devices/0000\:00\:03.0/rom >> cat /sys/bus/pci/devices/0000\:00\:03.0/rom > /dev/null >> ] > > Can't see why it reflects real world scenario (or a real world > scenario with same characteristics regarding kvm_mmu_zap_all vs faults)? > > Point is, it would be good to understand why this change > is improving performance? What are these cases where breaking out of > kvm_mmu_zap_all due to either (need_resched || spin_needbreak) on zapped > < 10 ? When guest read ROM, qemu will set the memory to map the device's firmware, that is why kvm_mmu_zap_all can be called in the scenario. The reasons why it heart the performance are: 1): Qemu use a global io-lock to sync all vcpu, so that the io-lock is held when we do kvm_mmu_zap_all(). If kvm_mmu_zap_all() is not efficient, all other vcpus need wait a long time to do I/O. 2): kvm_mmu_zap_all() is triggered in vcpu context. so it can block the IPI request from other vcpus. Is it enough?